Note: Descriptions are shown in the official language in which they were submitted.
~:97SSl
Backqround Of The Invention
This invention relates to electronic article surveillance
systems using magnetic phenomena and, in particular, to markers,
methods and apparatus for use in such article surveillance
systems.
Electronic article surveillance systems in which magr.etic
markers are used to detect the presence of articles under
surveillance are well known in the art. French patent No.
763,681 to Picard discloses an early system of this type. The
Picard patent teaches that low coercive force, high permeability
metals, such as permalloy, when subjected to an alternating
magnetic field induce harmonics which distinguish these metals
from other magnetic metals. These metals with their unique
harmonics can thus be used as magnetic markers to identify
objects which carry the markers.
Since the early days of the Picard patent, substantial
effort has been expended in an attempt to improve the existing
markers. This effort, for the most part, has been directed at
finding new materials having a lower coercive force and higher
permeability than was previously used. Because the voltage
pulse generated by the presence of the marker is dependent on
the hysteresis characteristic of the magnetic material of the
marker, by using materials with lower coercive force and higher
permeability, higher order harmonics with higher amplitude
' values could be realized for lower values of applied field,
551
thereby making the markers more unique.
While the search for ma~erials with higher permeability and
lower coercive force was thus the direction of most researchers,
a radically different approach is presented in U.S. Patent No.
4,660,025, entitled "Article Surveillance Magnetic Marker Having
i an Hysteresis Loop With Large Barkhausen Discontinuities", and
assigned to the same assignee hereof. In the '025 patent, a
magnetic marker is disclosed which does not depend upon a high
permeability, low coercive force material. Furthermore, the
output pulse developed in response to the presence of the marker
is substantially independent of the time rate of change of the
interrogating field and the field strength as long as the field
strength exceeds a low minimum threshold value. More,
particularly, the '025 patent teaches that by forming the marker
so that the magnetic material of the marker retains stress, the
marker exhibits a hysteresis characteristic having a large
Barkhausen discontinuity. Accordingly, upon exposure to an
interrogating field exceeding the low threshold value, the
magnetic polarization of the marker undergoes a regenerative
reversal. This so-called "snap action" reversal in the magnetic
I polarization results in the generation of a sharp voltage pulse,
,,1 rich in high harmonics, which affords a more unique detectable
signal.
11 In addition to the highly advantageous harmonic content and
¦ pulse output of the marker of the '025 patent, the marker is
~, - 2 -
~Z97~iS~
also advantageous in that it allows for deactivation by a n~mber
of techniques. These techniques are disclosed in U.S. patent
No. 4,686,516, entitled "Method, System and Apparatus for
Article Surveillance", and also assigned to the same assignee
hereof. More particularly, the '516 patent discloses one
practice for deactivating the marker of the '025 patent in which
the amorphous material of the marker is crystallized. This is
accomplisihed by heating at least a portion of the marker above
the crystallization temperature, either by application of an
electric current or radiant energy such as laser light. Another
procedure disclosed in the '~16 patent and useable with this
type of marker involves the application of mechanical or radiant
energy means to relieve the internal stress in the marker.
While some of these deactivation procedures enable deactivation
without touching the marker, they also require careful
application of the deactivation energy so that the energy is not
blocked from reaching adjacent articles.
It is therefore a primary object of the present invention to
provide an improved magnetic marker for electronic article
surveillance systems wherein the marker undergoes snap action or
step changes in its magnetic flux at low threshold values of the
applied field, while also being hands-off (i.e., non-contact)
deactivatable by simple means.
It is a further object of the present invention to provide 2
method of making the aforementioned improved magnetic marker.
"
-- 3 --
~ ~297S5~ 1
It is still a further objec:t of the present invention to
pxovide an electronic article surveillance system incorporating
the aforementioned improved magnetic marker.
It is yet a further object of the present invention to
provide an electronic article surveillance system incorpoarting
both deactivation means and the aforementioned improved magnetic ¦
marker.
Summarv of the Invention
In accordance with the principles of the present invention
the above and other objectives are achieved in a marker
comprising a magnetic material or means which is conditioned to
have a hysteresis characteristic of preselected character.
Specifically, when subjected to an alternating magnetic field,
the magnetic flux of the material undergoes a regenerative step
change at a threshold value when the field increases to the
threshold value from substantially zero and undergoes a gradual
change when the field decreases from the threshold value to
substantially zero. For field values below the threshold, there
is substantially no change in the magnetic flux of the material. ¦
In the present illustrative form of the invention, the
aforesaid characteristic is realized by conditioning the marker
magnetic material to having a domain structure of preselected
character. In particular, the domain structure of the magnetic
material i such that it remains unchanged, i~e., the domain
-- 4
~;~9755~
!
jj walls are in a pinned state, correspondiny to a demagnetized or
neglible flux condi~ion of the magnetic material, for increasing
magnitudes of applied field up to the aforementioned threshold
il value. At this threshold, the pinned walls become released,
¦¦ i.e., snap from their pinned condition, causing the flux of the f
Il magnetic material to undergo a regenerative step change in
¦I value. As the magnitude of the applied field is subsequently
decreased below the threshold value, the flux is gradually
decreased to the demagnetized or negligible flux condition and
the domain walls are-returned to their pinned state.
Due to the step change in flux of the magnetic material, the I
, marker of the invention induces perturbations in an applied
' interrogation field which are rich in high harmonics and which
!
are relatively independent of the field, analogous to the marker
of the '025 patent. Furthermore, because the marker depends on
step changes in flux to generate perturbations to the field, the ¦
marker can be deactivated by means which causes the step changes
to be replaced by gradual changes. I
¦ In the aforementioned, pinned domain wall form of the
¦ invention, this deactivation can be easily realized by further
conditioning which significantly diminishes the ability of the
domain walls to return to and remain in their pinned state. In
accordance with the practices disclosed herein, such further
conditioning is realized by application of a deactivating
magnetic field whose frequency and/or amplitude are
- 5 -
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71~76-~
substankially higher than the respec~ive frequency and/or
amplitude of the interrogating field.
In a further aspect of the present invention a method of
making or conditioning the aforesaid marker of the invention to
have the desired pinned wall domain configuration is disclosed. In
yet further aspects of the inven~ion, an electronic article
surveillance system and method incorporating the marker and
deactivation means for the marker are also disclosed.
In accordance with the present invention there is
provided a marker for use in an article surveillance system in
which an alternating magnetic interrogation field is established
in a surveillance zone and an alarm is activated when a
predetermined perturbation to said field is detected, said marker
comprising a magnetic means having a hysteresis characteristic
with a step change in magnetic flux such that upon subjecting the
magnetic means to an applied alternating magnetlc field, the
magnetic flux of the magnetic means undergoes a regenerative step
change in maqnetic flux at a threshold value when the field
increases to the threshold value from ~ubstantially zero and
undergoes a gradual change in magnetic flux when the field
decreases from the threshold value to substantially zero, the
magnetic flux of the material undergoing substantially no change
in flux value for increasing values of field below the threshold
value.
In accordance with the present invention there is also
provided a method of making a marker, the marker to be used in an
~ ~ 5
~2975~i1
71576-2
article surveillance system and being comprised of a maynetic
means, the method comprisiny the steps of: developing for the
magnetic means dsmains having a wall configuration; and annealing
said magnetic means .such that said wall configuration of said
domains remains in a pinned state for values of applied field
below a threshold value.
In accordance with the present invention there is
further provided a method for detecting the presence of an article
in an interrogation zone comprising the steps of: generating an
alternating magnetic interrogatlon field in the interrogation
zone, the magnitude of said interrogation field in said
interrogation zone exceeding a threshold value; securing a marker
to said article, the marker comprising a magnetic means having a
hysteresis characteristic with a step change in magnetic flux such
that upon subjecting the magnetic means to an applied altarnating
magnetic field, the magnetic flux of the magnetic means undergoes
a regenerative step change in magnetic flux at a threshold value
when the field increases to the threshold value from substantially
zero and undergoes a gradual change in magnetic flux when the
field decreases from the threshold value to substantially zero,
the magnetic flux of the material undergoing substantially no
change in flux value for increasing values of field below the
threshold value; and detecting perturbations of the interrogation
field in said interrogation zone when said marker is present in
said interrogation zone.
In accordance with the present invention there is
6a
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.,
129'7S51
71576-2
prov.tded a system for detecting the presence of an article in an
interrogation zone comprising: means for generating an alternating
magnetic in~errogation fleld in the interrogation zone, the
magnitude of said interrogati.on field in said interrogation zone
exceeding a threshold value; a marker secured to an article, the
marker comprising a magnetic means having a hysteresis
characteristic with a step change in magnetic flux such that upon
subjecting the magnetic means to an applied alternating magnetic
field, the magnetic means undergoes a regenerative step change in
magnetic flux at a threshold value when the field increases to the
threshold value from substantially zero and undergoes a gradual
change in magnetic flux when the ~ield decreases from the
threshold value to substantially zero, the magnetic flux of the
material undergoing substantially no change in flux value for
increasing values of field below the threshold value; and means
for detecting perturbations to the interrogation field in said
interrogation zone when said marker is present in said
interrogation zone.
In accordance with the present invention there is also
provided a method of deactivating an article surveillance marker,
the marker comprising a magnetic material having domains whose
wall configuration is of a character that in the absence of
deactivation the configuration enables the marker to be responsive
to an applied alternating magnetic interrogation field for causing
an associated article surveillance system to render an output
alarm, the method comprising: disabling the character of said wall
6b
p~
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71576-2
configuration o~ said domains.
In accordance with the present invention there is
further provided a marker for use in an article surveillance
system in which an alternating magnet:Lc interrogation fleld is
established in a surveillance zone and an alarm is activated when
a predetermined perturbation to said field is detected, said
marker comprising a maynetic means having when is substantially
demagnetized condition corresponding to a negligible flux domains
whose wall configuration is in a pinned state and remains in a
pinned state for increasing magnitudes of applied field up to a
threshold value at which the wall configuration is released from
the pinned state causing a regenerative step change in the
magnetic flux, the wall configuration of the domains returning to
the pinned state upon the magnitude of applied field being
decreased below the threshold value to a value resulting in said
demagnetized condition whereby said flux is gradually decreased to
the negligible flux.
Description Of The Drawinas
The above and other features and aspects of present
invention will become more apparent upon reading the following
detailed description in conjunction with accompanying drawings, in
which:
FIG. 1 shows a tag incorporating a magnetic marker in
accordance with the principles of the present invention;
FIG. 2 illustrates, in simplified form, the magnetic
domain configuration of the marker of FIG. 1 for various values of
.~. 6c
~Z~7S~;l
71576-28
applied field;
E'IG. 3 sho~s the hysteresi.s characteri~ic of the mark_r
of EIG. 1,
FIG. ~ shows the process steps for making the marker of
FIG. 1;
FIG. 5 illustrates the hysteresis characteristic of a
continuous length of magnetic material useable in forming the
marker of FIG. 1.
6d
~2~75~1
FIG. 6 illustrates the hysteresis charaeteristic of the
magnetie material of FI~. 5 after the material has been cut into
lengths suitable for the marker of FIG. 1, but prior to being
conditioned in accordance with the method of the invention and
FIG. 7 illustrates an eleetronie artiele surveillanee system
ineluding a deaetivation unit alnd incorporating the marker of
FIG. 1.
Detailed Deseri~tion
In ~IG. 1, a tag 1 in aecordance with the principles of the
present invention is shown. The tag 1 comprises a substrate ll
and an overlayer 12 between whieh is disposed a magnetie marker
13 comprising a magnetie material. The undersurfaee of the
substrate 11 can be eoated with a suitable pressure sensitive
adhesive for securing the markex 13 to an article to be
maintained under surveillance. Alternatively, any other known
arrangement can be employed to secure the marker 13 to the
article.
In aecordance with the invention, the magnetic marker 13 of
the tag 1 is conditioned so as to have a hysteresis
characteristic of preselected charaeter. More specifieally, in
aceord with the present illustrative form of the invention, this
eharaeteristic is realized by conditioning the marker to exhibit
a predetermined or preselected domain strueture with pinned
domain walls when the marker is in a demagnetized or negligible
flux eondition. This domain strueture is retained by the pinned ¦
walls for magnitudes of applied field up to a threshold value
_ 7
1297551
at which time the pinned walls release and the structure
abruptly changes causing a corresponding regenerative step
change or transition in flux. As the magnitude of the applied
field subsequently decreases below the pinning threshold to a
val-~e which again results in a neglible flux or demagnetized
condition, the domain structure returns to its equilibrium
state where the domain walls are again pinned.
FIG. 2 illustrates pictorially in A-E a simplified domain
structure for the marker 13 and how the structure changes with
applied field. FIG. 3, in turn, illustrates the resultant
desired hysteresis characteristic. In the simplified domain
structure of FIG. 2, a single domain wall 13c extends along the
length of the marker 13 initially centrally of its width to
define equal size domains 13a and 13b. However, in actual
practice, the domaln structure can take on any desired shape,
although structures having relatively simple domain walls of
long length are considered preferable. The hysteresis ~
characteristic in FIG. 3 is also pictorial in nature and no
attempt has been made to draw the characteristic to scale or in
scale proportions.
As can-be appreciated from FIGS. 2 and 3 J in the initial
demagnetized condition of the marker 13 (depicted in A of FIG.
2), the magnetic polarizations of the initialIy equal size
domains 13a, 13b of the marker 13 are of opposite first~and
second dlre t ons (hereinafter~referred to as the "positive" and
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12~75~1
"negative "directions, respectively) resulting in a
substantially negligible flux. As the applied magnetic field
increases in the positive direction, the domain wall 13c
separating the domains 13a, 13b remains unchanged or pinned in
position so that the neglible flux condition persists as
evidenced by the portion a of the hysteresis characteristic of
FIG. 3. When the field reaches the positive pinning threshold
+Hp, however, the wall 13c abruptly releases shifting to the
left so that the positive direction polarization domain 13a
becomes larger than the negative direction polarization domain
13b. This causes the marker 13 to abruptly take on an overall
positive magnetic polarization and to thereby result in a step
positive change in the magnetic flux. Curve portion _ in Fig
depicts this and shows that the flux has undergone a step
transition or jump at the applied field of +Hp to a positive
flux +Bp near the positive saturation +Bs~
Reduction of the applied field below the positive pinning
threshold +Hp now causes the flux to gradually decrease
(i.e., undergo a smooth "transformer like" characteristic) to a
neglible flux condltion corresponding to the demagnetized state
of the marker 13, as evidenced by curve portion c in FIG.3.
During this time, the domain wall 13c, which is no longer
pinned, gradually returns to its original pinned position or
site to again become pinned, as shown in C of FIG. 2, causing
the domains 13a,13b to also take on their original shape. As
I',
_g_
I ~297551
the applied field is now reversed in direction, the wall 13c of
the marker 13 remains pinned and the demagnetized or neglible
flux condition again persists as shown by the curve portion d in
FIG.3. Upon reaching the negat:ive pinning threshold -Hp, the
wall 13c abruptly releases, this time shifting to the right
causing the negative direction polarization domain 13b to be
enlarged relative to the positive direction polarization domain
13a (D of FIG. 2). The m~rker thus abruptly takes on an overall ¦
negative direction polarization, thereby causing the flux to
undergo a negative step transition or change as can be seen by
the curve portion e of FIG. 3. The flux thus takes on a
negative value -Bp close to the negative saturation value
-Bs~ Decrease of the negative field then causes a gradual
decrease of the flux along curve portion f in FIG. 3 to the
demagnetized or neglible flux condition and the wall 13c of the
marker 13 again returns to its pinned state as shown in E of
FIG. 2.
The pinning threshold value Hp evidenced by the marker 13
is established during conditioning of the the marker and,
preferably, is less than about 1.0 oersted. It is also
preferable that the demagnetizing field of the marker 13 be less
than about,~.O oersted and, more pre~erably, be within a range
of 0.5 to 0.8 oersted. The lower limit desired for the
demagnetizing field ensures that the effects of the earth's
magnetic field on the marker are minimized, while the upper
limit ensures that the drive of the applied field is within
-- 10 -
1~97S5~
acceptahle limits. It is further preferable for optimum
operation that the demagnetizing field be equal to or sligh~ly
less than the pinning thresholcl Hp.
The demagnetizing field of the marker 13 is the field which
arises in the marker in opposition to the applied field and is a
result of the finite length of the marker. Before the magnetic
material forming the marker 13 is cut into lengths suitable for
the marker, the material exhibits a hysteresis characteristic 51
as shown in FIG. 5. This characteristic evidences no
demagnetizing field for the material. Once the material is cut
into finite lengths, however, the hysteresis characteristic
tilts as shown by curve 61 in FIG. 6 evidencing a demagnetizing
field HDM which is determined by the intersection of the
dotted line 62 with the extrapolation of 61. Subsequent
conditioning of the marker material, as will be described below,
to realize the domain configuration described above for the
marker 13, results in substantially the same demagnetizing field
HDM but with the altered hysteresis characteristic for marker
depicted in FI~. 3.
Control over the demagnetizing field of the marker 13 to
achieve the field values discussed above can be realized by
varying the shape of the marker. Markers with dimensions of 5
centimeters in length, 2 millimeters in width and a 2i3 microns
in thickness have resulted in a demagnetizing field of 0.5
oersted. Conditioning of these markers has also resulted in a
pinning threshold of substantially the same value. It is
~Z97~1 7]576-28
believed that ribbons having a 2 inch length, a 0.25 inch width
and a 28 micron thickness could result in a demagnetizing field
and pinning threshold o-f about twice this value, i.e., of about
1.0 oersted. Thus, the markers of the invention, while long and
narrow, will likely not be required to be as extreme in length
as the markers of conventional tags in present use.
As can be seen from the above, the marker 13 of the
invention, due to its unique domain wall character and correspond-
ing hysteresis characteristic, exhibits step flux transitions at
relatively low values of applied field, i.e., less than about l.0
oersted. These step transitions will result in perturbations in
an applied field which will generate a sharp voltage pulse, rich
in high harmonics, which affords a more unique detectable signal
analogous to the signals realized witn the marker of the '025
patent.
The magnetic material of the marker 13 can be any
material or combination of materials which exhibit the hysteresis
characteristic of FIG. 3. Thus, crystalline magnetic materials,
such as Permalloy if adapted in this manner may be used.
Similarly, amorphous magnetic materials adapted in this manner
may also be used. Furthermore, while non-magnetostrictive
amorphous materials would be preferable, certain positive
magnetostrictive materials might also be useable.
Amorphous materials of the following compositions have
- 12 -
1;~97551
e~hibited the desired pinned wall properties:
C72 .15Fe5. ssSi5B15Mo2 (w)
Co7s 2Fe4 8Si2B18 (X)
Co74~26Fe4~74si3Bl8 (Y)
Co74~24Fe~.76si2Bl9 (Z)
In a marker formed from the composition (Y) above, the marker
exhibited a demagnetizing field of 0.3 oersted, a pinning
threshold of O.S oersted and a saturation field of 1.0 oersted.
As indicated above, the conditioning or fabrication
procedure for the marker 13 of the tag 1 enables the marker to
exhibit the desired domain wall and hysteresis properties
discussed above. FIG. 4 illustrates the steps in the
conditioning process or method. Magnetic marker material from a
supply, is first formed into a continuous body by a standard
forming procedure. This procedure will be dictated by the shape
desired for the marker, i.e., whether the marker is to have the
shape of a ribbon, wire, sheet, film or some other shape. The
magnetic marker material as supplied is usually free of spurious
domain structure caused by local strains and imperfections. If
spurious domain structure is found to exist at this point the
material can be heated, i.e., preannealed, to achieve a strain
free material.
The continuous body after forming is cut lnto lengths
desired for the particular markers belng ~abr~cated. The marker
lengths are then further processèd to develop the desired domain
1297551
configuration. This configuration is then fiYed in the markers
by annealing and the annealed markers are then cooled to
complete the process.
, The step of developing the desired domain configuration in
! each marker can be achieved in a variety of different ways. One
technique is to subject the marker to a varying magnetic field
and then to either slowly decrease the field or slowly remove
~ the marker from the field to demagnetize the marker. This will
j create a domain structure in the marker corresponding to a
demagnetized, negligible flux condition and the particular
structure can be tailored by adjusting the shape of the marker
and/or the application of the applied field. If the domain
structure is created in this manner, the subsequent annealing
'I and cooling steps are required to be carried out in a
¦ substantially field free enviorment. This, in turn, requires
that the envirornment be shielded from the earth's magnetic
field or, if shielding is not possible, that the earth's field
be balanced out.
Another technique for developing the desired domain
structure is to apply a magnetic field to the marker and hold
the field a~d marker in a fixed relationship which is continued
through the subsequen~ annealing and cooling steps. Thus, the
mar~er and a group of magnets can be held in a jig, for example,
to provide the desired configuration. The jig can then be
placed in the annealing equipment and the cooling equipment so
f'
.
. . '.
1~97~5~
that the domain configuration is maintained, while the
configuration is being fi~ed in the ribbon.
As mentioned above, FIG. 6 illustrates the hysteresis
characteristic of the marker 13 material after it has been cut
to length, hut prior to development of the desired domain
structure. As is apparent, the marker exhibits the normal
hysteresis with no step transitions in flux. After development
of the domain pattern and annealing to fix the pattern, the
hysteresis changes to that shown in FIG. 3, as above-described.
The temperatures and time periods suitable for the annealing
step in the conditioning of the marker 13 will depend upon the
factors surrounding the particular situation. Markers have been
fabricated with temperatures of 300 degrees C over time periods
of 20 minutes, 30 minutes and 1 hour and with temperatures of
400 degrees C over time periods of 30 minutes. A useable range
of temperatures and time periods might be 250-500 C and 30
seconds to 5 minutes. Of course, the annealing temperature must
be less than the Curie temperature and, if the magnetic material
is amorphous, also less than the crystallization temperature.
While, as discussed above, the marker 13 of the invention
l is advantageous in developing high harmonics which are
l! relatively independent of the applied field for low values of
the field, the marker is further advantageous in that it can be
readily deactivated without the need to physically touch ~he
i marker- In accord with the invention, this can be accomplished
il ,
Il - 15 - ,
~1 1
lZ97551
by subjecting the marker 13 to means which charlges the step flu
transitions in the marker hysteresis characteristic to gradual
changes. In the present illustrative form of the invention,
this is realized by means which prevents the domain walls of the
marker from returning to their pinned equilibrium state or sites
as the applied field is decreased to the demagnetized, negligble
flux condition. In further accord with the invention,
deactivation is preferably achieved simply b~ applying a
deactivating field to the marker which is adjusted in magnituZe
and/or frequency to disrupt or break up the domain configuration j
so the domain walls are unable to find their pinning sites.
Again, as with the conditions for annealing, the particular
frequency and/or amplitude of the field required to deactivate
the marker 13 will depend upon the factors attendant each
situation. However, the lowest deactivation frequency and/or
amplitude should be at least sufficiently greater than the
frequency and/or amplitude, respectively, of the field used for
interrogation that the latter can be accomplished without the
fear of deactivating the marker. For the marker in the example
discussed hereinabove, (i.e., the marker of (Y) composition with
the 0.3 oersted demagnetizing field) a deactivating field of 10
oersted was found sufficient to deactivate the ribbon. Another
marker having the composition (Z) above and operating in an
interrogation field of 10 Hz. frequency was able to be
deactivated with an applied field of 1 kHz frequency
' ~;~97S~l
at oersted. By making the amplitude a~d/or
frequency of the deactivating field at least an order of
magnitude greater than the respective amplitude and frequency of I
the interrogation field proper operation is reasonably assured.
When the aforementioned hig~ frequency or high amplitude
field is applied to the marker 13, the marker is caused to
reverse magnetic polarity in a short time. In order to
accsmodate this, the domain walls of the marker are forced to
break up creating more walls to reverse more quickly. The
original wall configuration is thus destroyed. As a result, the
wall configurations in the flux states corresponding to the
characteristic positions where c and f reach the demagnetized or
negligible flux state in FIG. 3, no longer match the
configurations originally annealed into the marker. The walls,
therefore, do not find their pinning sites, thereby resulting in !
a hysteresis characteristic which is similar to the
characteristic prior to development of the pinned domain wall
configuration, i.e~j,a characteristic asl~ illustrated in FIG.
6. The marker, therefore, no longer provides a rich high
harmonic response and acts like a piece of normal magnetic
material.
FIG. 7 illustrates use of the tag-1 in an article
surveillance system provided with a deactivation unit. More
particularly, the system 51 includes an interrogation or
surveillance zone, e.g:, an~exit area of a store, indicated by
the broken lines at SZ. Tag 1~ having attributes similar to the
' ~2~75Sl
tag 1 of the invention is shown attached to an article in the
zone 52. The transmitter portion of the system comprises a
frequency generator 53 whose output is fed to a power amplifier
54 which, in turn, feeds a field generating coil 55. The latter
~coil establishes an alternating magnetic field of desired
frequency and amplitude in the interrogation zone 52. The
amplitude of the field will of course vary depending upon system
parameters, such as coil size, interrogation zone size, etc.
~owever, the amplitude must exceed a minimum field so that tags
in the zone 52 will under all conditions see a field above the
aforementioned pinning threshold. A typical minimum field is
about 1.2 oersted.
The receiving portion of the system includes field receiving
coils 56, the output of which is applied to a receiver 57. When
the receiver detects harmonic content in signals received from
coils 56 in a prescribed range and resulting from the tag lA,
the receiver furnishes a triggering signal to alarm unit 58 to
activate the alarm.
It should be noted that the receiver portion of the system
51 should have a response time which is sufficiently fast to
detect the tag lA before the marker is brought to locations in
the interrogation zone 52 where the level of the field may be
sufficient to deactivate the tag (e.g., locations closely
adjacent the generating coil 55). Upon such detection, the
system 51 can then adjust the transmitting portion to reduce the
field in order to avoid deactivation. Alternatively, the system
!~ I
~Z~75S~
can be maintained at its original field level so that
deactivation of the tag lA occurs after detection.
A second tag lB also having attributes similar to the tag 1
of the invention is shown on an article outside the
I interrogation zone 52 and therefore not subject to the
interrogation field established in this zone. An authorized
, checkout station includes a tag deactivation unit 59. The tag
. lB is to be deactivated by passage along path 61 through the
deactivating unit S9. Passage of the tag lB results in a
deactivated tag lC, which may now pass freely through the
interrogation zone 52 without acting upon the interrogation
field in a manner triggering the alarm 58.
: As can be appreciated the deactivation unit 59 may simply
comprise a magnetic field generator with a frequency and/or
amplitude sufficient to disable the pinned state of the domain
configuration of the tag lB to result in the deactivated tag lC.
It should be noted that the magnetic marker 13 can take on a
variety of shapes and configurations. Thus, the marker can be
in the form of a ribbon, wire, sheet, film or other
~ configuration.
¦~ As above indicated, the marker 13 of the invention can be of
shorter length than conventional markers, while providing a
¦ higher signal output. Moreover, by varying the size and shape
of the marker it can be readily adapted to accomodate a variety
of environments as well as a variety of different surveillance
l l
- 19 - 'I
l l
l l
( ~Z~7SS~
system parameters. These advantages coupled with the ability to
readily deactivate the marker without touching it make it
useable in a variety of applica~ions, including use in price
stickers on products.
I In all cases it is understood that the above-identified
¦l arrangements are merely illustrative of the many possible
~¦ specific embodiments which represent applications of the present
~1 invention. Numerous and varied other arrangements can readily
¦¦ be devised in accordance with the principles of the invention
' without departing from the spirit and s-cope of the invention.
~0
